Genetic and Phenotypic Overview of Mutations in COL4A1 and COL4A2: Insights from a French Cohort ^†

Introduction: Type IV collagen, predominantly composed of the COL4A1 and COL4A2 genes, is a crucial component of the basement membrane, essential for maintaining vascular integrity [1]. Pathogenic variations in these genes are associated with a wide phenotypic spectrum that can affect multiple organ systems, with clinical manifestations ranging from prenatal to adult onset [2,3,4]. The objective of this study is to provide an overview of COL4A1 and COL4A2 pathogenic variations within a large French cohort and to refine the current understanding of their frequency, molecular characteristics, and clinical implications.

Methods: Over an eight-year period (2016–2023), we identified 144 unrelated individuals carrying pathogenic variants in COL4A1 or COL4A2. These patients were referred to the French reference laboratory for rare cerebrovascular diseases, which is the only facility in France with a dedicated gene panel for the molecular diagnosis of familial cerebral small vessel disease. Referral indications included fetal or neonatal brain hemorrhages, clinical features suggestive of COL4A1/2-related collagenopathy, and adult cases with suspected monogenic small vessel disease (SVD) warranting SVD gene-panel testing. Detailed phenotypic data were retrospectively collected and analyzed based on clinical records provided by the referring physicians across France.

Results: Genetic diagnoses were established during adulthood for 66% of the 144 probands and during childhood for 22%. In 12% of cases, the diagnosis was made during the investigation of an in utero cerebral hemorrhage in a stillborn fetus. Among the 144 probands, 105 carried pathogenic COL4A1 variants and 39 had pathogenic COL4A2 variants.

Glycine substitutions represented the majority of variations (59%). When comparing glycine variants identified in our patients to those found in controls (gnomAD v3.1.2), we observed that variants in controls tend to cluster within the N-terminal region of the triple helix domain. In contrast, among our patients, variants in fetuses are predominantly localized in the C-terminal part, whereas adult variants are distributed throughout the entire domain. Disruptive variants constituted the second most frequent category, accounting for 20% of cases, followed by deletion/duplication events and pathogenic variations located in the 3′ untranslated region.

An adult-onset of the disease was reported in 26 out of the 59 probands carrying a glycine variant in COL4A1 (mean age at genetic diagnosis = 57 years old, range 22–74). Imaging markers of cerebral small vessel disease was reported in all but two, including white matter hyperintensities (88%), lacunes (58%), and microbleeds (50%). Forty-five percent of them had experienced a stroke, which was a brain hemorrhage in 50% (one-third occurring before the age of 30) and a lacunar stroke in 50% of cases. Cognitive and/or psychiatric symptoms were reported in half of them. Intracranial aneurysms (15%), porencephalic cysts (10%), and systemic involvement including renal and/or ocular features (50%), were more frequently reported in glycine variations located in the C-terminal region of the protein.

An adult-onset of the disease was reported in eight of the probands carrying a disruptive variant in COL4A1 (mean age at genetic diagnosis = 48 years old). Although limited in number, they appeared as severe as adult-onset glycine variants, with cerebral hemorrhage reported in 37%, intracranial aneurysms in 50%, and ophthalmological and/or renal manifestations in 50% of probands.

Compared to COL4A1, adult-onset COL4A2 mutated probands (18 glycine, and eight disruptive variants) showed a lower prevalence of neurological and systemic manifestations, although lacunar strokes and renal dysfunction were notably reported in 38% and 23% of cases, respectively.

Conclusions: This study illustrates the broad mutational and clinical spectrum of COL4A1/A2 pathogenic variations, along with the incomplete penetrance of their associated manifestations. It serves as the starting point for a standardized French national database, which is crucial for refining our understanding of the clinical risks associated with COL4A1/A2 variations, ultimately improving patient management and genetic counseling.